Remote monitoring system



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United States Patent 3,327,289 REMOTE MONITORNG SYSTEM Hallan Eugene Goldstine, Port Jefferson, Ezra Cohen, Brooklyn, and .lack Henry Wolff, West Nyack, NSY., and Charles G. Arnold, Chatham, NJ., assignors to Radio Corporation of America, a corporation of Delaware Filed .lune 7, 1963, Ser. No. 286,366 14 Claims. (Cl. 340-151) This invention relates to systems for monitoring a plu rality of remote stations from a central or terminal station and, particularly, to a system for selectively interrogating a plurality of remote stations from a terminal station in a manner to provide at the terminal station a direct analogue measurement of a condition subject to variation in each of the remote stations.

Various applications are known in which remote monitoring systems are provided for maintaining a check on the performance of equipment physically spaced from a central or terminal point. By way of example, remote monitoring systems, defined in the art as fault finding or alarm systems, are used in microwave relay communication systems to monitor and report failures in the operational status of unattended repeater stations. Alarm systems are also used to remotely ch-eck the pressure at spaced points along a pressurized communication cable system and to monitor the performance of pipe line and electric power line systems.

Alarm systems of the type described include an attended terminal station at one or both ends of the system and a number of unattended remote stations or check points spaced from one another at distances determined by the requirements of the particular application. In a typical installation, some sort of two-position switch mechanism is provided at each of the remote stations. The switch mechanism may be responsive to temperature, pressure, velocity, acceleration, humidity or other conditions. When a condition at the remote station to be monitored is satisfactory, the switch mechanism is unoperated. When the condition becomes unsatisfactory as determined -by the threshold of the switch mechanism, the switch mechanism is operated. The terminal station is connected to the remote station over an alarm system which permits personnel at the terminal station to determine the status of the switch mechanism at the remote station. Since the terminal station can determine only whether the switch mechanism is operated or unoperated, such an alarm system is limited as to the information which it provides.

The terminal station then can determine only a simple on-off indication from the remote station. The operation of the switch mechanism may result from a sudden and severe failure, requiring immediate, emergency attention. On the other hand, a gradual deterioration may have occurred which can be corrected during routine maintenance procedures. Since the terminal station has no way of knowing which situation exists, the worst case is assumed. Emergency maintenance is then provided for the remote station when, in fact, such attention may not actually be needed.

A further problem which exists in the use of an alarm system as described above is that the switch mechanism itself can fail or be accidentally operated to provide an erroneous indication of the condition being monitored. The switch mechanism may break-down in a manner such that a change in the monitored condition is indicated at the terminal station when in fact no such change in the condition has occurred. Also, the switch mechanism may not respond to a change in the monitored condition, providing no indication of that change at the terminal station. Since the terminal station can not determine whether an indication of failure received from a remote station is due to an actual failure in the monitored condition or only to a failure in the switch mechanism, the same emergency procedures must be followed in either case.

In spite of the above disadvantages and others, alarm systems using two-position (operated-unoperated) switch mechanisms have been employed in the art because such systems represented the most economical and practical approach available. A need exists -for a simple and relatively inexpensive alarm system which will provide at a terminal station a measurement which is directly proportional to the change in a monitored condition at each of a plurality of remote stations, avoiding the limitations of a two-position switch mechanism for indicating the status of a monitored condition and the attendant problems.

It is an object of the invention to provide an improved remote monitoring system.

Another object is to provide an improved remote monitoring system in which there is made available at a terminal station a direct, analogue measurement of a condition subject to Variation in each of a plurality of remote stations.

A further object is to provide an improved remote monitoring system by which a plurality of equipments can be selectively interrogated from a central point in a manner to make available at the central point a direct, analogue measurement of a condition subject to variation in each of the equipments.

A still further object of the invention is to provide an improved alarm system by which the actual amount of change in a condition subject to variation in each of a plurality'of remote stations is monitored at a terminal `station with the particular remote station monitored being selectable from the terminal station.

The objects are accomplished according to one embodiment of the invention by a system arrangement including a terminal station connected over a single pair of lines or conductors to a plurality of remote stations. Each of the remote stations includes a transducer of the type arranged to provide a resistance which varies substantially linearly and directly with a given parameter or condition.

A remote station upon being interrogated from the terminal station functions to connect the resistance of the transducer across the lines. The resistance of the lines between the terminal station and the interrogated remote station is known. By subtracting at the terminal station the voltage drop due to the known resistance from that due to the total line resistance, a voltage value results which is directly proportional to the resistance of the transducer at the remote station and, therefore, the value of the condition to which the transducer is responsive. This voltage value is recorded in any suitable manner, providing a running account of the monitored condition as the remote station is interrogated during successive time periods. By interrogating the remote stations one at a time from the terminal station either manually or automatically, a measurement directly proportional to the value of the monitored condition at first one and then another of the remote stations is made available at the terminal station.

One embodiment of a remote monitoring system constructed according to the invention is shown in the block schematic circuit diagram comprising the single figure of the drawing.

A terminal station TS includes a constant direct current (D.C.) source 10. The source 10 may include any direct current source such as a battery with current regulating means, for example, a constant current diode, for maintaining the current substantially constant regardless of load conditions. By Way of example, the source 10 may be arranged to supply a current of 1 milliampere with a tolerance of i3 percent. The positive side of the D.C.

source is connected to the output terminal 11 over a path including lead 12 and inductor 13. The negative side of the D.C. source 10 is connected to a second output terminal 14 over a path including lead 15, a resistor 16 and inductor 17. The terminal station TS can be considered as including three separate circuit arrangements connected between the output terminals 11 and 14. They are a remote station interrogating circuit indicated generally as reference numeral 18, a line resistance compensation circuit indicated generally as reference numeral 19, and a recording circuit indicated generally as reference numeral 2G.

The remote station interrogating circuit 18 includes an oscillator 21 from which a lbasic single frequency tone is applied to a plurality of mixer stages 22, 23 and 24 over a lead 40. The mixer 22 combines the frequency of the oscillator 21 with that of an oscillator 25 to produce a single frequency tone of frequency F1 which is applied to a contact 26 on a switch 27. The output of a further oscillator 28 is applied to the second mixer 23. The frequency of the oscillator 28 is determined so that the mixer 23 combines the outputs of the oscillators 21 and 28 to form a single frequency tone of frequency F2. The single frequency tone produced by the mixer 23, which is of a frequency different from that of the tone produced by the mixer 22, is applied to a Contact 29 on the switch 27. In much the same manner, the mixer 24 combines the output of the oscillator 21 and the output of a still further oscillator 30 to produce a single frequency tone of frequency FN. The tone generated by the mixer 24 is applied to the contact 31 on the switch 27. The frequency of the tone provided by the mixer 24 is different from that generated by the mixers 22 and 23.

The switch 27 in addition to the contacts 26, 29 and 31 includes a wiper arm 32 and an open contact 33. The wiper arm 32 is arranged for rotation between the contacts 33, 26, 29 and 31 in the direction of the arrow 34. The wiper arm 32 of the switch 27 is connected through an amplifier 35 to the primary winding 36 of a transformer 38. One end of the secondary winding 37 of the transformer 38 is connected through a capacitor 39 to the output terminal 11, the other end of the secondary winding 37 being connected to the second output terminal 14. The remote station interrogating circuit 18, thusly, includes a plurality of tone generators of different frequencies with means in the form of a switch 27 for selectively applying the tone signals to the output terminals 11 and 14.

The line resistance compensation circuit 19 includes a second constant direct current (DC.) source 45. The negative side of the second D.C. source is connected to the junction of the resistor 16 and inductor 17 over a lead 41, with the positive side of the second DC. source 45 being connected to the junction of the resistor 16 and the negative side of the rst D.C. source 10 over a lead 42. A rst resistor 46 is connected between the negative side of the second D.C. source 45 and a contact 47 on a second switch 48. A second resistor 49 is connected between the negative side of the second D.C. source 45 and a contact 50 on the switch 48, with a third resistor 51 being connected between the negative side of the second D.C. source 45 and a contact 52 on the switch 48. The switch 48 includes a wiper arm 53 and an open contact 54. The wiper arm 53 is arranged for rotation between the contacts 54, 47, 50, and 52 in the direction of the arrow 55. As indicated by the dashed line 56, the wiper arm 32 of the switch 27 and the wiper arm 53 of the second switch 48 are preferably ganged together by any suitable mechanical means for simultaneous operation. The line resistance compensation circuit 19 acts to place varying amounts of resistance in parallel with the resistor 16 in accordance with the setting of the wiper arm 53.

The recording circuit 20 of the terminal station TS includes a scale divider potentiometer. The potentiometer includes a resistance element 59 connected between the positive and negative sides of the rst DC. source 10. A chart recorder or other recording means 60 is connected between the wiper arm 61 of the scale divider potentiometer and the negative side of the first D C. source 10. An alarm circuit 62 is also connected to the Wiper arm 61 and may include a light, a lamp or other alarm arrangements.

The two output terminals 11 and 14 at the terminal station TS `are connected over a pair of lines 57 and 58, respectively, to a rst remote station R1. The remote station R1 may be positioned at any desired distance from the terminal station TS and is located at a point where a condition is to be monitored by the terminal station TS. A rst electrical path is completed at the remote station R1 between thc lines 57, 58 including inductor 63, capacitor 64, the primary winding 65 of a transformer 66, capacitor 67, and inductor 68. One end of the secondary winding 69 of the transformer 66 is connected to the line `57 with the other end of the secondary winding 69 being connected to the anode of a crystal diode 71 poled for current conduction in the direction of the arrow. The diode 71 may be of the type identified as 1N458. A capacitor 70 is connected across the secondary winding 69.

The cathode of the diode 71 is connected directly to the base electrode of a rst planar, NPN transistor 73, for example, of the type identified as 2N1711 and manufactured by the Radio Corporation of America. A resistor 74 and a capacitor 75 are connected in parallel between the cathode of the diode 71 and the line 57. The emitter electrode of the transistor 73 is connected directly to the base electrode of a second planar, NPN transistor 76 which may be of the same type as the rst transistor 73. A temperature stabilization resistor 77 is connected between the emitter electrode of the rst transistor 73 and the line 57. The emitter electrode of the second transistor 76 is returned directly to the line 57.

The collector electrode of the first transistor 73 is connected to the collector electrode of the second transistor 76 over a path including a current limiting resistor 78 and a further resistor 79. The junction of the resistors 78 and 79 is connected to one end of the resistance element 80 in a potentiometer-type transducer 81. The other end of the resistance element 80 is connected to the line 58. Therefore, in addition to the rst electrical path completed between the lines 57, 58 and referred to above, a second electrical path is completed between the lines 57, 58 including the resistance element 80 of the transducer 81 and the two parallel-connected transistors 73, 76.

The lines 57, 58 are shown as extending to a second remote station R2 and a third remote station RN. The remote stations R2 and RN are each connected across the lines 57, 58 and include circuitry substantially identical to that of the rst remote station R1 shown in detail. A potentiometer-type transducer capable of translating the value of a changeable condition directly into a resistance value is included in each of the remote stations R2 and RN. The transducer 81 in the remote station R1 and the transducers in the remote stations R2, RN may all be designed to respond to the same type of condition or may be designed to respond to different types of conditions. By way of example, potentiometer-type transducers suitable for use in the remote stations R1, R2, RN are available commercially that respond to such conditions as temperature, velocity, pressure, acceleration, humidity, as well as other conditlons. As long as the transducer provides a variable resistance, it may be responsive to any one of a number of possible conditions yas required in the particular application.

In describing the operation of the remote monitoring system, it will rst be assumed that the wiper arm 32 of the switch 27 engages open contact 33 and the wiper arm 53 of the second switch 48 engages open contact 54. The remote station interrogation circuit 18 is disconnected from the lines 57, 58 so that no tone is applied to the remote stations R1, R2 and RN. The D.C. source 10 is activated to supply a constant current, for example, of l milliampere (ma.) to the lines 57, 58. The source preferably includes voltage limiting so that the voltage output of the power supply is limited to a value slightly greater than the maximum operating voltage. Capacitor 39 serves to isolate the primary winding 37 of transformer 38 of the remote station interrogating circuit 18 from the D.C. source `10.

The transistors 73 and 76 in the first remote station R1 being of the planar variety are characterized by low leakage, high current gain at low input signal levels and high impedance in the off or non-conducting state. The application of the constant current to the lines when the remote stations are inoperative causes the voltage at the constant current supply to limit at a predetermined value. This voltage, which is slightly more than the expected maximum operating voltage, appears on the lines 57, 58 and provides voltage on the collectors of the transistors 73, 76 but no voltage on the base electrodes. The transistors 73, 76 are back-biased or non-conducting. The high impedance of the transistors 73, 76 acts to effectively disconnect the transducer 81 from across the lines 57, 58. Similarly, the transducers at the second and third remote stations R2, RN are held open-circuited from across the lines 57, 58.

Assuming that it is now desired to interrogate the remote station R1 for an indication of the condition monitored by the transducer 81, the mechanical linkage 56 at the terminal station TS is operated to cause the wiper arm 32 to engage contact 26 of the switch 217 and the Wiper arm 53 to engage contact 47 of the switch 48. Resistor 46 is now placed in parallel with the resistor 16. At the same time, the single frequency tone of frequency F1 is applied to the lines 57, 58 over the path including amplifier 3-5, transformer 38 and the output terminals 11, 14. Inductances 13, 17 serve to prevent the appearance of the tone across the D.C. source 10.

The remote station R1 includes a filter with a balanced input consisting of two series tuned LC circuits represented by inductor 63-capacitor -64 and inductor 68- capacitor 67. The lter accepts the single frequency tone of frequency F1 and rejects all other frequency signals. The other two filters of remote stations R2 and RN reject all frequencies except F2 and FN respectively. The secondary of the transformer 66 is als(` tuned for additional selectivity and provides voltage step-up at the resonant frequency F1.

The rectifier consisting of diode 71, resistor 74 and capacitor 75 converts the alternating current signal to a direct ycurrent level applied to the base electrode of the transistor 73 sufcient to drive the transistors 73 and 76 into saturation. In this condition, the resistance from the collector to emitter electrodes of the transistors 73, 76 becomes low, effectively connecting the resistance element 80 of the transducer 81 directly across the lines 57, 58. The resistor 79 in the collec-tor circuit of the transistor 76 serves three functions. Resistor 79 serves to bring the total line resistance between the terminal station TS and the remote station R1 up to a level such that any variation in the resistance due to temperature or other uncontrolled variable is relatively small. Secondly, when the transistors 73 and 76 begin to switch state, the collector voltage lwill dropa tending to reduce the current gain of the transistor 73, thereby impeding the switching action. Resistor 79 serves to maintain a voltage across the transistor 73 throughout the switching sufiicient to ensure a high current gain by the transistor 73, facilitating the rapid and complete change of the transistors 73, 76 from their non-conducting states to the saturated or conducting states. Thirdly, resistor 79 serves to limit the current thru the transducer resistor 80 .and transistor 76 due to any high Voltage surges, for example.

The resistance of the transducer 81 having been placed across the lines 57 and 58, the voltage appearing on the lines 57, 58 becomes proportional to the variable resistance of the transducer 81 plus the resist-ance of lines 57, 58 between the terminal station TS and the remote station R1. That is:

where Vm equals the voltage; I,3 equals the constant current; RF equals line resistance plus any fixed resistance in series with the line other than the resistance of the transducer 81; and RT equals the resistance of the potentiometer-type transducer 81.

The total line resistance less the resistance of the transducer 81 is substantially constant and is known by measurement. Resistor 46 is made of a value so that the combination of the resist-ance provided by the resistor 46 and that provided by resistor 16, placed in series with the lines 57 and 58, is equal to the known resistance RF. The value of the resistor 46 is determined according to the specific line resistance from the terminal station TS to the remote station R1. By connecting the second D.C. source across the resistor `46 with the polarity shown and a current rating larger than that of the first D.C. source 10, for example, 2 ma., there is provided, in effect, a power supply in series with the lines 57, 58 which develops a Voltage drop across the combination of resistors 46 and 16 equal in magnitude but opposite in polarity to the voltage appearing on the |lines due to the known resistance RF. The quantity ICRF is subtracted from the above equation, leaving the voltage appearing on the lines 57, 58 directly proportional to that due to the resistance of the transducer 81 and therefore an indication of the existing value of the condition to which the transducer 81 is responsive.

The recorder is responsive to the voltage appearing on the lines 57, 58 to provide a permanent record. The scale of the recorder is made to read in units of the condition being monitored by the proper setting of the wiper arm 61 in the scale divider potentiometer. The alarm 62 functions to alert maintenance personnel if any recorded value is less than a predetermined value. The alarm circuit can be a voltage sensitive relay adjusted for a specific value which actuates a bell or a lamp.

The operation of the other remote stations R2 and RN is similar to that of the first remote station R1. When it is desired to interrogate the second remote station R2, the wiper arm 32 of the switch 27 is made to engage contact 29 and the wiper arm 53 of the switch 48 engages contact 50. A single frequency tone of frequency F2 is applied -to the lines 57, 58 from the mixer 23. Since the filter input of the first remote station R1 is responsive only to the frequency F1, the base voltage is removed from the transistors 73, 76 which again become nonconducting. The high impedance of the transistor 76 is placed in series with the resistance element 80, electrically removing the transducer `81 from across the lines 57, 58, The fi-lter input of the second remote station R1 is responsive only to the frequency F2. Upon the application of the tone signal of frequency F2 to the lines 57, 58, the remote station R2 functions to place the resistance element of the transducer included therein across the lines with the operation being substantially identical to that described in connection with the remote station R1.

Resistor 49 in the line resistance compensation circuit 19 is determined to provide in co-mbination with the D.C. source 45 and the resistor 16 a voltage drop equal in magnitude but opposite in polarity to the voltage on the lines 57, 58 due to the known resistance in the lines 57, 58 between the terminal station TS and the second remote station R2. The resulting voltage appearing on the lines 57, 58 is directly proportional to the resistance of the transducer included in the remote station R2 and, therefore, the value of the condition at the remote station R2 monitored by that transducer. The recorder 60 is operated to record the voltage in units of the monitored condition, providing a permanent record of the direct measurement of the monitored condition.

To interrogate the third remote station RN, the wiper arm 32 of switch 27 at the terminal station TS is made to engage Contact 31, with the wiper arm 53 of switch 48 being made simultaneously to engage contact 52. A single frequency tone of frequency FN is applied to the lines 57, 58 from the mixer 24, and resistor 51 is placed in parallel with the D.C. source 45 and the resistor 16. The first and second remote stations R1 and R2 both operate to maintain the transducers therein effectively disconnected from the lines 57, 58.

The third remote station RN includes a filter input responsive only to the frequency FN. The resistance element of the transducer included in the third remote station RN is connected across the lines 57, 5S. Resistor 51 is of a value so that the resulting voltage drop provided by the combination of the resistors Sland 16 and the D.C. source 45 cancels the voltage on the lines 57, 58 due to the known resistance in the lines 57, 58 between the terminal station TS and the third remote station RN. The resulting voltage is directly proportional to the resistance of the transducer in the third remote station RN and, therefore, the existing value of the monitored condition. This voltage is recorded by the recorder 60 in units of the monitored condition.

Resistor 16 in the line resistance compensation circuit 19 at the terminal station Ts is of a value to provide continuity in the line as the wiper arm 53 of switch 48 passes from one contact to another. Where a type of switch is used for the switch 48 which makes-beforebrakes, the resistor 16 can be removed with continuity being provided through the switch 48.

A remote monitoring system is provided for selectively interrogating a plurality of remote stations from a terminal station, providing at the terminal station a direct, analogue measurement of a condition monitored at each of the remote stations. An operator at the terminal station can determine whether a change in a monitored condition at one of the remote stations has occurred suddenly and with a severity necessitating immediate action. Where the change is a gradual one as indicated by successive readings of the recorder at the terminal station, the operator can determine how soon maintenance will be required or, in fact, whether any action other than that involved in the usual maintenance procedures is needed. The operator is readily able to determine whether an emergency actually exists and the severity of the situation.

This is in contarast to a system where a two-position switch is used at a remote station to indicate a conditions status such that the condition must be considered as good or bad, there being no indication as to how good or how bad. To offset this limitation, it is common practice to set the thresholds of the two-position switches higher than is actually required. This is done to provide some warning sufficiently in advance of a bad condition to permit the dispatch of maintenance personnel. Since the worst case must be assumed, the operator is able to exercise little judgement in maintaining the system. Such a precaution is not needed in the operation of the described system. The operator can determine from the direct measurement available the nature of the change in the monitored condition and exactly when maintenance personnel must be dispatched. A more ecient operation results.

Since the operation described uses the presence of the transducer resistance across the lines, any failure in the operation of the remote station such that the transducer resistance is not properly placed across the lines is readily traceable as being due to failure in the remote station itself rather than to a change in the monitored condition. Even under the extreme limits of the monitored condition, a path must be completed through lines 57, 58 and the transducer resistance element. An open circuit in either the transducer itself or in the circuitry by which the transducer is connected across the lines is clearly indicated at the terminal station by the open line. An operator is not likely to mistake a failure of the remote station for a change in the monitored condition.

A feature of the invention is the simple and compact construction of the remote station, permitting a small, light-weight construction which lends itself to the use of micro-modular and similar techniques. A separate power supply is not required at the remote station, all operating voltages being derived from the terminal station via the connecting lines. Only a single pair of lines is needed for both the interrogation and information read-out.

In the installation of the remote monitoring system, the remote stations are placed at the desired locations where some condition is to be monitored. The remote stations are connected together and to the terminal station by a pair of lines in the manner shown in the drawing. Resistance measurements are made from the terminal station to each remote station on a station by station basis with the transducer resistance removed at each remote station. The value of the compensation resistors to be included in the line resistance compensation circuit are determined from the measurements. To facilitate the proper alignment of the system, the compensation resistors, resistors 46, 49 and 51 in the drawing, may be variable, permitting ne adjustments of the compensating resistances. After installation, all adjustments and alignments are made at the terminal station.

While only three remote stations are shown in the illustrated embodiment of the invention, any number of remote stations may be used according to the needs of a particular application. As indicated by the dotted line section of the switch 27 and lead 40, a tone generator consisting of a mixer and oscillator and connected to a contact of the switch 27 is added at the terminal station for each additional remote station. A separate resistor connected between lead 41 and a contact on the switch 48 in the line resistance compensation circuit 19 is also added for each additional remote station, as indicated by the dotted line section of the switch 48 and lead 41. Since the remote stations are responsive to the polarity of the D.C. source 10, it is possible to double the number of remote stations included in the system without doubling the number of tone generators. An arrangement is provided for reversing the polarity of the D.C. source 10 across the lines 57, 58. Two remote stations are assigned to the frequency of each tone generator included in the remote station interrogating circuit 18. one of the two remote stations is responsive to the tone signal when the D.C. source 10 is connected with one polarity across the lines 57 and 58, the other remote station being responsive to the same tone signal when the D.C. source 10 is connected across the lines 57, 58 with the opposite polarity. If terminal stations are located at both ends of the lines 57, 5S and if sucient attenuation exists for frequencies transmitted over the lines in opposite directions from the terminal stations, it is possible for the terminal stations to provide the same interrogating frequency signals, reducing the total number of different frequency signals used in the system.

A pair of parallel-connected transistors 73, 76 have been described as a preferred arrangement for selectively connecting the resistance element of the transducer at each remote station across the lines 57, 58. However, any arrangement may be used in place of the parallel-connected transistors which will operate at the available signal power to provide a high impedance in one state and a low impedance in a second state. A single transistor may be used if it has a sufficient current gain. Silicon PNPN controlled rectiers, as well as other semiconductor devices, have been successfully employed in such applications.

A particular type of remote station interrogating circuit 18 has been shown and described in connection with the illustrated embodiment of the invention. The invention is not limited to the particular circuit shown. For example, one may employ any suitable arrangement for selectively producing a number of different frequency tone signals. Instead of using a separate one of the oscillators 25, 28, and 36 along with the basic oscillator 21 for each tone, a single oscillator may be used. Suitable means can be provided in the form of variable capacitance or inductance for changing the frequency of the oscillator as the wiper arm of the switch in the remote station interrogating circuit is moved from contact to contact. In addition, different oscillators each having the frequency to address a different one of the remote stations may be used.

The two switches 27 and 48 in the terminal station TS are shown as being manually operated by a mechanical linkage 56. Suitable means may be provided for automatically cycling the switches 27 and 48 so that the remote stations are each periodically interrogated in a desired order and for predetermined time periods. The switching function can be accomplished mechanically, electro-mechanically, or electronically using semiconductor devices or tubes.

A single recorder 60 is shown as being shared by the remote stations. A separate recording means can be provided for each remote station with means to switch between the recording means as first one and then another remote station is interrogated. The use of a separate recording means is desirable where different types of conditions are monitored at the respective remote stations, requiring each recording means to respond in units of the particular monitored condition.

The operation of the system described above is to be contrasted to that of a system in which a voltage source is used in place of the constant current source. By using a constant current source and monitoring voltage, the voltage is proportional to the current times the transducer resistance. If plotted, the monitored voltage versus the transducer resistance is linear with respect to the resistance read-out. That is, a change in the transducer resistance results in an equal change in the monitored voltage. If a constant voltage source is used and current is monitored, the current is an inverse function of the transducer resistance. When plotted, the monitored current Versus the transducer resistance is non-linear with respect to the resistance read-out. A change in the transducer resistance results in an unequal change in the monitored current. By using a constant current source, it is possible to provide a direct measurement of the transducer resistance at the terminal station, simplifying the operation of the system. The use of a constant voltage source requires additional complication to compensate for the resulting non-linearity mentioned above. In addition, when current is constant the voltage drop due to line and fixed resistance is constant and may be determined and subtracted. If a constant voltage source were used, the voltage drop due to line and fixed resistance would not be constant but would vary with the current as a function of transducer resistance. Therefore, cornpensation for line and fixed resistance would be more complex.

In a pressurized communication cable system where it is desired to monitor cable pressure at spaced points, a typical cable system can be forty miles in length. Twenty remote stations may be spaced at approximately two mile intervals along the cable. The remote stations can be built into the cable or may take the form of a unit secured to the outer layer of the cable with pressure tube and electrical connections into the cable. The lines by which `the terminal station is connected to the remote station can be included as a cable pair within the cable, avoiding the use of external lines. The remote station interrogating circuit at the terminal station can be arranged tovprovide a plurality of tone signals within the range of 30G-5,000 cycles per second, for example, the frequencies being spaced at intervals of ten percent of the center frequency thus conserving spectrum.

A potentiometer-type pressure transducer is included in each of the remote stations and is arranged to provide a resistance proportional to the cable pressure at the remote station locations. In a typical buried cable system,

the cable pressure is within the range of 0 to 10 lbs. per square inch gauge. One example of a potentiometer-type pressure transducer that may be used at the remote stations is identified as model .3071 (L-173) and is manufactured by the Servonic Instrument Inc. This unit monitors pressure over a range of 14.7 to 24.7 pounds per square inch absolute, providing a resistance varying over the range 0 to 10,000 ohms directly with pressure. The scale of the recorder 60 or recorders at the terminal station Ts are proportioned by the scale divider potentiometer 59, 61 to read in units of pressure. The operation of the system in providing a direct measurement at the terminal station Ts of the monitored pressure at each of the remote stations is as described above. A recording of the monitored pressures is possible with a high degree of accuracy.

In the typical operation of a cable plant, routine measurernents of pressure at various points along the cable are made. This has been done manually so that the requirement represents a high maintenance cost factor. This expense involved in the operation of a cable plant is eliminated by the system described herein, since direct readings of the pressure at the remote points are always available at the terminal station. When a cable leak or fault occurs, it has been necessary for field maintenance personnel to make manual pressure measurements over an extended period of time. The measurements are used to provide for the plotting of pressure gradients along the cable and the determination of the location of the cable leak. Once the cable leak location is determined, normal repairs can be made. By making it possible to obtain and plot the pressure readings remotely, the invention serves to reduce the expense and complexity of cable repair procedures.

What is claimed is:

1. A system for providing at a terminal station a direct indication of a condition at each of a plurality of remote stations where said condition is subject to varia- -tion over a range of values comprising, ohrnic circuit means connecting said terminal station and kall of said remote stations, a separate condition-response impedance transducer means at each of said remote stations with said transducer means each having an impedance which varies with the condition to which the transducer means is responsive, means at said terminal station for transmitting one of a plurality of signalling waves of different frequencies to all of said remote stations over said circuit me-ans, means at each of said remote stations for selectively receiving a given one of said signalling waves of a frequency which is individual to that one of said respective remote stations, means at eaoh of said remote stations to rectify waves lof said individual frequency, means at each remote station responsive to said rectifying means to vconnect said impedance transducer means in said connecting circuit means, and means at said terminal station to provide an indication which is directly proportional to the impedance of s-aid impedance transducer means, whereby an indication of the impedance of a selected one of said impedance transducer means and therefore an indication related to the conditions of the respective units is obt-ained.

2. The system as recited in claim 1 in which each of said impedance transducer means is a potentiometer translducer means, with said potentiometer transducer means each being arranged to provide a resistance which varies directly with a condition to which the potentiometer transducer means is responsive.

3. In combination, a first station, a plurality of other stations spaced at different distances from said first Station, a single pair of lines connecting said other stations in parallel to said first station, a separate transducer means at each of said other stations with said transducer me-ans each being arranged to provide a resistance which varies with a condition to which the transducer means is responsive, and means at said first station for concurrently supplying over said lines a station selecting signal and a predetermined current to all of said other stations for selectively operating one of said other stations Iover said lines to connect said operated other station transducer means to said lines to provide at said first station a voltage which is directly proportional to t-he resistance of the selected one and then another of said transducer means 4and therefore related to the value of the conditions to which said Irespective transducer means are responsive.

4. In combination, a first station, a plurality of otherl stations spaced lat different distances from said first station, a single pair of lines connecting said other stations in parallel to said first station, a separate transducer means at each of said other stations with said transducer means each being arranged to provide a resistance which varies directly and linearly with a condition to which the transducer means is responsive, means at said first station for concurrently -applying over said lines a station selecting signal and a predetermined current to all of said other stations for selectively operating one of said other stations over said lines, means to connect the resistance of said transducer means of the selected station across said lines with a voltage appearing at said first station on said lines with is directly proportional to the resistance of said respective transducer means, and means at said first station for recording said voltage in units of the condition to which said respective transducer means are responsive.

5. An arrangement for providing at a terminal station a direct indication of the pressure existing at other stations at spaced points along a pressurized cable system comprising, a single pair of lines connecting said other stations in parallel to said terminal station, a separate pressure t-ransducer means at each of said other stations with said transducer means each being arranged to provide a resistance which varies linearly with the pressure to which that transducer means is responsive, means at said terminal station for transmitting one of several distinct signals over said lines to all of said other stations, means at each of said other stations to select a different one of said distinct signals, means at each of said other stations responsive to said selecting means to place the resistance of the corresponding one of said transducer means across Said lines, means at said terminal station for applying a constant current over said lines, and means at said terminal station for measuring the voltage appearing on said lines, and means at said terminal station for recording said voltage in units of pressure.

6. A system for providing at a termina-l station a direct indication of a condition at each of a plurality of spaced remote stations where said condition is subject to variation over a range of values comprising, a pair vof lines connecting said terminal station to each of said remote stations, said lines having different resistances between said terminal station and each of said remote stations, a separate transducer means at each of said remote stations with said transducer means each being arranged to provide a resistance which varies with a condition to which the transducer means is responsive, means at said terminal station for selectively applying a plurality of different actuating signals to said remote stations over said lines, each of said remote stations including means responsive only to a particular one of said actuating signals to connect the resistance of the transducer means in that one of the remote stations across said lines, means at said terminal station for applying a constant direct current to said lines, and means at said terminal station for applying a compensating Voltage to the resultin-g voltage appearing on said lines at said terminal station to compensate for said different line resistances thereby providing at said terminal station a voltage which is directly proportional to the resistance of the transducer means connected across said lines and therefore related to the Value of the condition to which said last-mentioned transducer means is responsive.

7. In combination, a first station, a plurality of other stations spaced at different distances from said first station, a pair of lines connecting said first station yand said other stations, said lines having different resistances hetween said first station and each of said other stations, a separate transducer means at each of said other stations with said transducer means each being arranged to provide a resistance which varies linearly with a varying condition to which the transducer means is responsive, means at said first station for selectively applying a plurality of different frequency tone signals to said other stations over said lines, each of sai-d other stations including means responsive only to a particular one of said signals to connect the resistance of the transducer means in that one of said other stations across said lines, means at said first station for applying a constant direct current to said lines, means for applying a compensating voltage to the resulting voltage appearing on said lines at said terminal station to compensate for said different line resistances thereby providing a voltage which is directly proportional to the resistance of the one of said transducer means connected across sai-d lines and therefore related to the value of the condition to which said lastmentioned transducer means is responsive, and means at said first station for recording said voltage in units of the condition to which said respective transducer means are responsive.

8. A combination as claimed in claim 7, and wherein, said means in said other stations responsive to said signals includes a switch connected in series with said transducer means across said lines and a filter, said filter being responsive only to a particular one of said signals to operate said switch to connect said resistance of said transducer means across said lines for the duration of said one signal.

9. In combination, first and second stations, a single pair of lines connecting said first and second stations, said lines having a predetermined resistance, a series circuit including a transducer means and a switch means connected across said lines at said second station with said transducer means being arranged to provide a resistance in said series circuit which varies with a condition to which the transducer means is responsive, means at said first station for applying a constant direct current to said lines, means at said first station for applying an actuating signal over said lines to said second station concurrently with said constant direct current, means connected across said lines at said second station responsive to said signal to actuate said switch means to complete said series circuit across said lines for direct current conduction for only the duration of said signal, and means at said first station for applying a compensating voltage to the resulting voltage appearing on said lines to compensate for said predetermined line resistance thereby providing a voltage which is directly proportional to the resistance of said transducer means and which is therefore related to the value of said condition at the same time said signal is applied over said lines.

10. A system for providing at one point a direct indication of a condition at a second point where said condition is subject to variation over a range of values comprising, a single pair of lines connecting said first and second points, a series circuit includng a transducer means and a switch connected across said lines at said second point with said trans-ducer means being arranged to provide a resistance in said series circuit which varies with said condition, signal generating means at said first point for applying an actuating signal over said lines to said second point, means connected across said lines at said second point responsive to said signal to actuate said switch to complete said series circuit across said lines for current conduction, means at said first point for applying a constant direct current to said lines, means at said first point for providing on said lines a voltage drop equal in magnitude but opposite in polarity to the voltage appearing on said lines due to the total resistance in said lines Ibetween said iirst and second points less the resistance of said transducer means so that there appears at said rst point a voltage directly proportional to the resistance of said transducer means and therefore related to the value of said condition, and means at said rst point for indicating said voltage in units of said condition.

11. A system as claimed in claim 10, and wherein, said signal generating means applies a single frequency tone signal as said actuating signal over said 4lines to said second point, said signal responsive means at said second point including a filter connected in parallel with said series circuit across said lines and responsive only to said tone signal to actuate said switch to complete said series circuit for current conduction across said lines.

12. In combination, a first station, a plurality of other stations spaced at different distances from said first station, a single pair of lines connecting said other stations to said first station, said lines having different resistances between said lirst station and each of said other stations, a separate series circuit including a transducer ymeans and a normally open switch means connected across said lines at each of said other stations with said transducer means each arranged to provide a resistance in said series circuit which varies directly with a condition to which the transducer means is responsive, means at said iirst station for selectively applying one of a plurality of different actuating signals to said lines, a separate control means connected across said lines at each of said other stations with said control means each being responsive only to a particular one of said actuating signals to actuate said selected switch means to complete a path for current conduction through said series circuit and across said lines, means at said first station for applying a constant current to said lines, means at said first station for applying a compensating voltage to the resulting voltage appearing on said lines upon one of said series circuits being completed across said lines to compensate for said different line resistances thereby providing a voltage which is directly proportional to the resistance of the particular one of said transducer means included in said last-mentioned series circuit, and means at said first station for indicating said further voltage in units of the condition to which said one transducer means is responsive.

13. A system for providing at a terminal station a direct analogue indication of a condition at a plurality of remote stations where said condition is subject to variation over a range of values comprising, a single pair of lines connecting said terminal station to said remote stations, a separate transducer means at each of said remote stations with said transducer means each being arranged to provide a resistance which varies linearly with a condition to which the transducer means is responsive, means at said terminal station for selectively applying a plurality of dierent frequency tone signals to said remote stations over said lines, each of said remote stations including means responsive to only one of said tone signals to connect the resistance 4of the transducer means in that one of said remote stations across said lines so that the resistance of only one of said transducer means is connected across said lines at a time, means at said terminal station for applying a constant direct current to said lines, means at said terminal station for providing upon the resistance of one of said transducer means being connected across said lines a voltage drop on said lines equal in magnitude but opposite in polarity to the voltage appearing on said lines due to the total resistance between said terminal station and that one of said remote stations including said one transducer means less the resistance of said one transducer means, said providing means serving to provide a voltage on said lines at said terminal station which is directly proportional to the resistance of said one transducer means and therefore to the value of the condition to which said one transducer means is responsive,

14. An arrangement for providing at a monitoring station a direct indication of the pressure existing at space-d points along a pressurized cable system comprising, a single pair of lines connecting said station to said spaced points, a separate pressure transducer means at each of said spaced points with said transducer means each being arranged to provide a resistance which varies linearly with changes in the pressure to which the transducer means is responsive, means at said station for selectively applying a plurality of different frequency tone signals to said spaced points over said lines, means at each of said spaced points responsive only to a particular one of said tone signals to connect the resistance of the transducer means at the same spaced point across said lines so that the resistance of only one of said transducer means is connected across said lines at a time, means at said station for applying a constant direct current to said lines, means at said station for providing upon the resistance of one of said transducer means being connected across said lines a voltage drop on said lines equal in magnitude but opposite in polarity to the voltage appearing on said lines due to the total resistance between said station and that one of said spaced points including said one transducer means less the resistance of said one transducer means, said providing means serving to provide a voltage on said lines at said station which is directly proportional to the resistance of said one transducer means and therefore to the value of the pressure to which said one transducer means is responsive, and means at said station for indictaing said last-mentioned voltage in units of pressure.

References Cited UNITED STATES PATENTS 1,995,594 3/1935 Wunsch S40- 180 3,083,357 3/1963 Chapin 340-180 FOREIGN PATENTS 1,203,305 1/1960 France.

NEIL C. READ, Primary Examiner.

THOMAS B. HABECKER, Examiner. 

1. A SYSTEM FOR PROVIDING AT A TERMINAL STATION A DIRECT INDICATION OF A CONDITION AT EACH OF A PLURALITY OF REMOTE STATIONS WHERE SAID CONDITION IS SUBJECT TO VARIATION OVER A RANGE OF VALUES COMPRISING, OHMIC CIRCUIT MEANS CONNECTING SAID TERMINAL STATION AND ALL OF SAID REMOTE STATIONS, A SEPARATE CONDITION-RESPONSE IMPEDANCE TRANSDUCER MEANS AT EACH OF SAID REMOTE STATIONS WITH SAID TRANSDUCER MEANS EACH HAVING AN IMPEDANCE WHICH VARIES WITH THE CONDITION TO WHICH THE TRANSDUCER MEANS IS RESPONSIVE, MEANS AT SAID TERMINAL STATION FOR TRANSMITTING ONE OF A PLURALITY OF SIGNALLING WAVES OF DIFFERENT FREQUENCIES TO ALL OF SAID REMOTE STATIONS OVER SAID CIRCUIT MEANS, MEANS AT EACH OF SAID REMOTE STATIONS FOR SELECTIVELY RECEIVING A GIVEN ONE OF SAID SIGNALLING WAVES OF A FREQUENCY WHICH IS INDIVIDUAL TO THAT ONE OF SAID RESPECTIVE REMOTE STATIONS, MEANS AT EACH OF SAID REMOTE STATIONS TO RECTIFY WAVES OF SAID INDIVIDUAL FREQUENCY, MEANS AT EACH REMOTE STATION RESPONSIVE TO SAID RECTIFYING MEANS TO CONNECT SAID IMPEDANCE TRANSDUCER MEANS IN SAID CONNECTING CIRCUIT MEANS, AND MEANS AT SAID TERMINAL STATION TO PROVIDE AN INDICATION WHICH IS DIRECTLY PROPORTIONAL TO THE IMPEDANCE OF SAID IMPEDANCE TRANSDUCER MEANS, WHEREBY AN INDICATION OF THE IMPEDANCE OF A SELECTED ONE OF SAID IMPEDANCE TRANSDUCER MEANS AND THEREFORE AN INDICATION RELATED TO THE CONDITIONS OF THE RESPECTIVE UNITS IS OBTAINED. 